DESCRIPTION: Moderate levels of physical activity are associated with a number of health benefits, including reduced risk of coronary heart disease, essential hypertension, diabetes mellitus, and colon cancer. There also is strong evidence that moderate physical activity reduces the risk of osteoporosis by increasing bone mineral levels. However, there is growing evidence that high levels of physical activity, particularly endurance-type activity (e.g., running, soccer, and basketball) causes reductions in bone mineral. Several recent cross-sectional studies have reported reductions in bone mineral among athletes engaged in high levels of activity (runners, cyclists, swimmers) compared to athletes engaged in less intense training. Recent prospective data collected in the investigators laboratory indicate that collegiate athletes engaged in high levels of endurance activity (basketball players) show substantial drops in bone mineral over time. Declines in bone mineral among young athletes are a major public health concern for two reasons. First, a low bone mineral level is a primary risk factor for stress fractures in athletes and young adults. Stress fractures among high school and collegiate athletes are highly prevalent, painful, and costly. Second, bone mineral among young athletes should be increasing during adolescence and young adulthood. Peak bone mass, which typically is not achieved until the third decade of life is a primary determinant of osteoporosis in later life. Thus, exercise-induced bone mineral loss could place adolescents at increased risk of osteoporosis in later life. Clearly, recent prospective findings of exercise-related bone mineral loss among young athletes need to be replicated in a larger, more heterogeneous cohort. The Specific Aims of this project are to: (1) conduct a two-year longitudinal study of bone mineral density (BMD) changes comparing male and female high-school athletes (basketball players) to age-, sex-, and race-matched non-athletes. (2) to identify risk factors for bone mineral loss among adolescent athletes, including gender, race, body composition, dietary calcium intake, reproductive hormone status, and training intensity.